Saiv-Flies. 173 



the part becomes first brown and then black, while it also 

 becomes more and more elevated. This increased elevation 

 is not owing to the growth of the bark, the fibres of which, 

 indeed, have been destroyed by the ovipositor-saw, but to the 

 actual growth of the egg ; for when a new-laid egg of the 

 saw-fly is compared with one which has been several days 

 enclosed in the groove, the latter will be found to be very 

 considerably the larger. This growth of the egg is contrary 

 to the analogy observable in the eggs of birds, and even of 

 most ether insects ; but it has its advantages. As it con- 

 tinues to increase, it raises the bark more and more, and 

 consequently widens, at the same time, the slit at the 

 entrance; so that, when the grub is hatched, it finds a 

 passage ready for its exit. The mother-fly seems to be 

 aware of this growth of her eggs, for she takes care to deposit 

 them at such distances as may prevent their disturbing one 

 another by their development. 



Another species of saw-fly, with a yellow body and deep 

 violet-coloured wings, which also selects the rose-tree, 

 deposits her eggs in a different manner. Instead of making 

 a groove for each egg, like the preceding, she forms a large 

 single groove, sufficient for about two dozen eggs. These 

 eggs are all arranged in pairs, forming two straight lines 

 parallel to the sides of the branch. The eggs, however, 

 though thus deposited in a common groove, are carefully kept 



Nest of eggs of Saw-fly, in rose-tree. 



each in its place ; for a ridge of the wood is left to prevent 

 those on the right from touching those on the left and not 

 only so, but between each egg of a row a thin partition of 

 wood is left, forming a shallow cell. 



The edges of this groove, it will be obvious, must be 



